Abstract Despite decades of research, curing glioblastoma (GBM) remains an intractable problem. Fresh approaches are needed and in this proposal we will use sex differences in GBM to generate novel insights into mechanisms that control gliomagenesis and treatment responses. Like many other cancers, GBM occurs more frequently in males than females, regardless of age or geographical location. The reason(s) for sex differences in cancer incidence and survival are largely unknown but must be traceable to the actions of sex chromosomes, and the long-term epigenetic and acute effects of sex hormones. Understanding how sex imparts affects cancer incidence and outcome will be essential for development of truly personalized precision medicine approaches to cancer treatment, as accounting for patient sex is a fundamental component of their individuality. In this proposal we will use sex differences to create a parallax-like effect, in which previously unapparent elements of GBM biology will be revealed. We recently used this approach and discovered cell intrinsic sex differences exist in thresholds for malignant transformation in a model of mesenchymal GBM. We hypothesize that fundamental mechanisms of cancer susceptibility and resistance lie within the biology of sex differences. To complete these studies we will build a new model system to examine sex differences in GBM. We have two specific aims. In the first aim we will adapt our model of mesenchymal GBM to support the analysis of how early events in sexual differentiation pattern responses to oncogenic events and establish different thresholds for transformation in male and female astrocytes. We will make use of the four- core genotypes model of sex differences and address hypotheses regarding how differences in sex chromosome complement and the epigenetic effects of sex hormones affect tumorigenesis. In the second aim we will use the same model to address the critical translational science objective of determining which of these developmental factors underlies sex differences in response to common and experimental chemotherapeutics. Together these studies constitute a pioneering approach to GBM biology. Success in these studies has real potential for novel insights with substantial translational significance as we strive to implement personalized precision approaches to treatment for GBM and other cancers.